The present invention relates to a process for imparting decarburization resistance to chrome-molybdenum steel. More particularly, the present invention relates to a process for obtaining a steel material suitable for use as a structural material of a sodium-heated steam generator in a liquid metal fast breeder reactor or of a nuclear fusion reactor in which lithium is used.
The term "chrome-molybdenum steel" herein indicates a steel corresponding to any of STPA24, STBA24 and SCMV4 specified in JIS (Japanese Industrial Standard) G3458, G3462 and G4109, respectively (see JIS Handbook, Steel (1979) published by Japanese Standards Association).
As heat-transfer pipe material of, for example, a sodium-heated steam generator in fast breeder reactor, there has been used a material obtained by annealing 2.25 Cr-1 Mo steel at 920.degree.-940.degree. C. and then furnace-cooling the same, or a material obtained by normalizing 2.25 Cr-1 Mo steel and then tempering the same. However, when such a material as thus treated is used in sodium at a high temperature, the carbon concentration of the material is reduced (decarburized). The decarburization rate is considerably high. Carbon incorporated in the material to form a solid solution so as to enhance the strength of the material, or carbon formed by the decomposition of carbides in the material, is dissolved into sodium and thus the strength of the material is weakened.
Various investigations have heretofore been made to reduce the decarburization rate and to enhance the decarburization resistance. For enhancing the decarburization resistance, it is effective to react carbon contained in the material in the form of a solid solution with iron, chromium, molybdenum and other minor constituents of the material to form stable carbides in the material. As a process for forming carbides in the material, heat treatment at a temperature of about 700.degree. C. for 0.5 to 3 hours has been proposed. However, according to the conventional heat treatment, the degree of reduction in the rate of decarburization is low, though the decarburization rate can be reduced to some extent. Further, carbide particles formed in the material by the conventional heat treatment become partially coarse. This is unfavorable from the viewpoint of the mechanical strength of the material at high temperature.